Marriage of workers (5)
I believe the answer is:
union
This clue might be a double definition.
'marriage' is the first definition.
(I know that marriage can be written as union)
'of workers' is the second definition.
The definition suggests a singular noun which matches the answer.
Can you help me to learn more?
(Other definitions for union that I've seen before include "See 22" , "The coming together of separate parts" , "Agreement -- wedlock" , "mating" , "Organisation of employees formed to bargain with the employer" .)